They are key telephones designed to be unplugged and replugged (so called "portable") by the subscriber when not in use, as opposed to permanently plugged in sets. They have a special engineering # stamped on the base. (GSE xxxxx)
The applications were outdoor phones near places where there was a swimming pool, or other weather affected areas, board rooms of corporations, or other spots where the subscriber did not need or want a full time installation. The plug was large enough not to incur damage from clumsy subscribers. The socket was a cast aluminum box, with mounting ears, approx 6" by 3" by 3" deep, with a spring loaded cover, e/w a gasket. The socket came factory wired on a length of 25 pair cable.
Since the plug/socket arrangement had only 16 pairs, some wiring compromises were necessary. Five PU require 10 pairs. (T,R,A,L) Then add the 5 lamp grounds, a buzzer pair and a ringer pair, and A1 ground requires total 15 pairs. One more spare for signaling if necessary. Like the man said, 4 yellow pairs and all 5 of the violet pairs were left out.
I had several NYTelCo subscribers with these phones, notably rich & famous in penthouses/terraces in Manhattan, where the phone was brought out and in by the staff. When a phone needed to be replaced in urban or high class suburban locations. because of damage from weather, it was a hassle to order these "special assemblies", as they were called. I Believe only the New York company offered them, but they may have been available in other places.
Towards the end of 1A2, I took a few of the male plugs off of damaged sets, used a Dremel to create a rectangular slot in the back of the plug, and pop riveted a 25 pair female amphenol connector onto the back of the plug. I then soldered the appropriate wires and made adapters, so a standard 565 or 2565 could be taken off the shelf and used immediately with no service delay. Alas, the end of 1A2 was upon us, so I never had a chance of submitting my design to Bell Labs, in the hopes of receiving a certificate, suitable for framing, for my amazing creation.
